#include<iostream>
#include<vector>
using namespace std;
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
struct TreeNode
{
    TreeNode* left; TreeNode* right;
};
class Solution {
public:
    int countNodes(TreeNode* root) {
        if (!root)return 0;
        int left = countNodes(root->left);
        int right = countNodes(root->right);
        return left + right + 1;
    }
};